Since the presence of the above impurities deteriorate the properties of the product resins prepared from bisphenol A, a close attention is paid for obtaining high-quality bisphenol A from the reaction product described above. Extensive researches have been done in order to obtain high-quality bisphenol A. A preferable process is that the condensation reaction liquor is fed into a crystallizer with a special construction after being concentrated by a concentration column, cooled to crystallize bisphenol A in the form of 1:1 (molar ratio) adduct crystal of bisphenol A with phenol, followed by centrifugal filtration and washing, to obtain the relatively pure adduct crystal, then bisphenol A is obtained by removing phenol from the adduct crystal.
In order to obtain high-purity bisphenol A by removing phenol from the adduct crystal of bisphenol A with phenol, researchers proposed various methods such as recrystallization, solvent extraction, distillation and steam stripping. For example, a recrystallization method by using water as solvent is disclosed in U.S. Pat. No. 4,212,997/1980 to General Electric Company. In this process, water of an amount of 8 to 10 times more than that of bisphenol A is added to the adduct, which is allowed to melt thoroughly at a temperature of 80.degree.-90.degree. C. The mixture is well stirred and cooled, and a prismatic crystal of bisphenol A is precipitated, which is filtered off with a purity up to 99.2% by weight, and a phenol content of less than 1% by weight. If a further purification is needed, water may be used or an organic solvent of bisphenol A insoluble or slightly soluble may be selected to wash the bisphenol A crystal. A method using methylene chloride as solvent is described in U.S. Pat. No. 4,156,098/1979 to the General Electric Company, wherein the adduct is dissolved at a temperature of no higher than 60.degree. C., then the temperature is lowered to 30.degree.-40.degree. C., and a bisphenol A crystal of relatively crude size is formed. After filtration, the crystal is washed with methylene chloride of an amount of twice as much as that of the adduct to obtain a white crystal of bisphenol A with a purity of 99.24%. The large amount of water or solvent introduced by the recrystallization method brings about a need for the recovery of a substantial amount of water or solvent, thus considerably increase the capital cost and operation cost.
A dephenolizing method often used is decomposing the adduct by heating in vacuum and distilling phenol at the same time. For example, U.S. Pat. No.3,936,507 discloses a process by vaporizing the adduct crystal at over 180.degree. C. for 0.1-30 minutes under reduced pressure and obtaining bisphenol A by fractional condensation. In such a distillation process, bisphenol A is heated at high temperature and easy to decompose, which produces new impurities and colored substances, thus makes it difficult to secure the desired high-purity and color level of polycarbonate grade bisphenol A. In general, there is also a need for a further procedure of hot water recrystallization as described in U.S. Pat. No. 4,209,646 or solvent washing as described in U.S. Pat. No. 4,492,807. As is noted in EP 0,343,349, phenol can not be completely removed only by distillation, a further procedure is needed, such as steam stripping as described in JP 43937/1972 or solvent recrystallization as described in JP 88137/1982. Accordingly, not only the stock is subjected to high temperature by using the dephenolizing method of distillation, but there is also a problem of a large amount of solvent recovery caused by its further procedure, as well as problem for treatment of a large amount of phonetic waste water caused by steam stripping method. So the whole procedure of refining bisphenol A becomes longer, which causes a substantial increase of capital cost and operation cost. The purity of bisphenol A can be increased by increase the capital and operation cost in some methods, however, due to the colored impurities caused by high temperature operation, the color of the product is difficult to be assured.
Also a process is disclosed in TOKKOSHO 36-23335 of heating the adduct to a temperature above 50.degree. C. in a solvent with a boiling point higher than 50.degree. C. and extracting only the phenol portion from the adduct. It also presents the above mentioned problems of solvent recovery and loss of bisphenol A from solvent dissolution.